ddPCR data graph

[naji]

Just wondering (from the BioRad 1-D ddPCR data analysis graph): why does the baseline of negative events sit at an amplitude of 1000 on the intensity graph? (the one plotting fluorescence intensity vs. droplet number, labeled as "channel 1 amplitude" vs "event number")? I understand that the red threshold line is what differentiates positive and negative events, but why do the negative events sit at 1000?

Thank you!